This invention relates generally to top and bottom pincushion correction circuits and specifically to such circuits that incorporate active vertical amplifiers having non-inverting and inverting inputs and pincushion correction apparatus that injects a correction current into the vertical deflection yoke.
As mentioned in the copending application, the advent of cathode ray tube (CRT) video monitors used in conjunction with computers for displaying alphanumeric and precise graphical data has created a need for closer control of the raster shape. The flat-faced color CRT has greatly increased the difficulty of pincushion correction. Active pincushion correction circuits and accurate deflection amplifiers, that closely control deflection currents, are ideally suited to pincushion correction. The deflection amplifier includes a feedback circuit that senses the current in the deflection yoke and develops a voltage that is fed to the inverting input terminal of the vertical amplifier for accurately controlling the deflection current. Such a vertical amplifier, when used in conjunction with the top/bottom pincushion correction circuit disclosed and claimed in the copending application, results in the top/bottom pincushion correction signal being detected by the sensing resistor in the feedback circuit for the vertical amplifier. The vertical amplifier "sees" the pincushion correction signal as an error signal and attempts to negate its effect. The vertical amplifier cannot follow the frequency of the pincushion correction signal and distortion is introduced. The distortion is not significant; yet, with the precise raster control desired in a flat tension mask CRT environment, for example, any distortion is undesirable.